A body, such as a vehicle, moving in a desired direction inevitably experiences motion in other directions as well. This undesired motion often arises from disturbances in the medium through which the body travels. For example, in a vehicle, whether one travels by land, sea, or air, one might encounter imperfections, bumps, waves, air pockets, and the like. At best, such random acceleration causes displacement, discomfort or annoyance to those in the body. This can also cause vibration and undesired horizontal or vertical movement to goods in the body. For certain susceptible individuals, these random accelerations can trigger a bout of motion sickness. However, in some cases, a particularly violent acceleration will cause the operator to briefly lose control of the body. Also, goods can be damaged when submitted to acceleration or shocks. Even when stationary, there may be some residual vibration associated with the vehicle's engine. In motion, even on smooth roads, this residual vibration can become tiresome.
A primary purpose of a body's suspension system is to provide vertical or horizontal compliance between the medium, such as the road, and the chassis, in order to isolate the chassis occupants or goods from the roughness in the road and to maintain the contact point(s) with the road, thus providing a path for transferring forces from the contact point(s) to the chassis. In applications where the body is a wheeled body, the contact point is also used to change the speed or direction of the body. In a wheeled body, examples of some common independent suspension linkages are known generally as strut & link (also called MacPherson strut), double A-arm (also called double wishbone or SLA), trailing arm, semi-trailing arm, multi-link, fork, scissor, pivot to name but a few.
In vehicles such as automobiles, each wheel assembly is connected to the chassis by one or more links. A link is defined as a substantially rigid member with a joint or joints at each end that allows a particular motion to take place. It is these links that control the motion (or path) of the wheel as it moves up and down over road bumps. These links also have to transmit the forces generated at the tire-road interface to the chassis. Rubber bushings are typically used at the ends of links to reduce the transmission of vibrations to the chassis. Unfortunately, the use of bushings also introduces compliance into the linkage that can be undesirable for accurately controlling wheel motion.
In an active suspension, controlled forces are introduced in the suspension, such as by hydraulic or electric actuators, between the sprung mass of the vehicle body and its occupants, and the unsprung mass of the wheel assemblies. The unsprung mass is the equivalent mass that reproduces the inertial forces produced by the motions of those parts of the vehicle not carried by the suspension system. This primarily includes the wheel assemblies, any mass dampers associated with the wheel assemblies, and some portion of the mass of the suspension links. The sprung mass is the mass of those parts of the vehicle carried by the suspension system, including the body. Active suspension systems may introduce forces that are independent of relative wheel motions and velocities.
Generally, all kinematically-induced wheel forces are either forces created by the interaction between the tires and the road, or inertial forces generated by the motion of the unsprung mass. The forces occurring between the tires and the road are transferred via the suspension system to the body. As long as the wheel assembly does not change its horizontal position or angular orientation relative to a smooth road surface, no substantial lateral or longitudinal tire forces (ignoring friction) will be created.
The tread width for a given pair of wheels is defined as the lateral distance between the centers of tire contact points with the road. When wheels bounce and rebound on most independent suspensions for passenger cars, the tread width changes. Solid axle suspensions (generally not favored due to ride quality and weight issues) and full trailing arm suspensions (generally considered only for rear suspensions on straight line racing vehicles, such as for drag racing), are generally immune from tread width changes. On commonly used passenger vehicle independent suspensions, such as the strut and SLA suspensions, the rather short suspension links pivot about fixed points or axes on the body, inboard of the wheel assemblies. The outer ends of the links (attached to the wheel assemblies) are therefore constrained to move in a substantially circular path with respect to the body, as viewed from either end of the vehicle. This link motion alters the position of the bottom of the tire relative to the body, changing the overall tread width.
Tread width change creates lateral forces, higher rolling resistance, and deterioration in directional stability of the vehicle. Conventional passenger cars typically have tread widths that widen with jounce travel and narrow during rebound. When a bump compresses both wheels of an independent axle simultaneously, the lateral forces applied to the vehicle body by lateral movement of one wheel tend to be balanced by lateral forces from the other wheel. Unfortunately, bumps seldom generate equal jounce and rebound on both sides of the vehicle simultaneously, and unequal wheel motions result in net lateral forces being applied to the vehicle body from tread width changes.
As undesirable as tread width change can be, configuring a conventional suspension to geometrically eliminate tread width changes tends to create unacceptable levels of roll during cornering, due to positioning of the theoretical “roll centers” of the front and rear suspensions. An excessive amount of roll is uncomfortable to the driver and passengers, can adversely affect tire grip, and uses up valuable suspension travel needed to avoid bottoming of the suspension on bumpy corners. The suspension roll center is a theoretical point in the center of the vehicle (viewed from the front) and in the center of the axle (viewed from the side) around which the vehicle body will rotate when subject to centrifugal force. It is also the point at which lateral forces can be viewed as effectively applied to the sprung body mass by the suspension. Therefore, the tendency for the vehicle body to roll is proportional to the distance between the roll center and the center of gravity of the body, and the optimum position of the roll center to minimize roll is at the height of the center of gravity of the sprung mass. However, the higher the roll center, the larger the tread width change. With the roll center above ground level, tread width will increase during jounce and decrease on rebound. It will be understood that the location of the roll center constantly changes with suspension position.
Wheel camber is another variable for tuning the characteristics of a vehicle suspension. Camber is the angle between the wheel center plane and a vertical to the plane of the road. Camber is positive when the top of the tire is inclined outwards away from the center of the vehicle, and negative when inclined inwards. When a vehicle is loaded to its design weight, a slightly positive camber value of, for example, 0.1 degree is considered ideal to keep the tires as upright as possible on the crowned road surface, for low rolling resistance and uniform tire wear. Many passenger car suspensions employ a static camber setting between about zero and negative 1.3 degrees, and effect dynamic camber change through selective suspension compliance, to offset negative static camber to try to provide a nearly zero camber on the outside tires in cornering. Some have said that keeping the camber near zero under all conditions is a primary goal of modern suspension systems, perhaps because tires are all designed to operate at a particular camber angle for optimum grip, and even small deviation from that angle can reduce tire grip capability.
When a vehicle with independent suspension is cornering, the wheels tend to tilt with the body. Thus, as the car body rolls toward the outside of the bend, the outside wheel goes into positive camber relative to the road, reducing its lateral grip. To combat this effect, many suspension linkages are designed to geometrically induce negative camber in jounce and positive camber as they rebound, even though such geometric camber adjustments will cause camber shifts during bumps as well as during cornering.
Cars with relatively soft suspensions will tend to pitch during braking and acceleration, dipping at the front and rising at the rear under heavy braking, and the opposite during hard acceleration. This pitching motion tends to put more strain on the neck muscles of the vehicle occupants than during simple linear acceleration and deceleration without such body rotation. Also, pitching motions are perceived to be objectionable by many passengers. Many suspensions incorporate anti-dive (to reduce forward pitching during braking) and anti-squat (to reduce rearward pitching during acceleration) configurations to reduce this effect.
An example of a simple anti-dive design is the use of a leading arm in the front suspension and a trailing arm in the rear suspension. With a leading arm, the effective arm pivot at the body is rearward of the effective arm pivot at the wheel. The relative locations of the effective pivots are reversed for a trailing arm. Under braking action, the calipers tend to rotate with the wheel producing an upward reaction at the front of the body and a downward force at the rear, producing an anti-dive effect. Traditional suspension linkages, like the double wishbone, can be designed with the wishbones pivot axes angled to give an effective leading arm length. However, such arrangements tend to induce undesirable wheel caster angle changes during jounce and rebound. Therefore, some manufacturers compromise by correcting only a percentage (typically, about 50 percent) of the brake live. Furthermore, an anti-dive geometry carefully calculated to match a particular front/rear brake force distribution will seldom give the perfect correction for anti-squat.
Improvements in suspension configuration are generally needed, particularly for use with active suspension control means.
Roll bars for motor vehicles are usually used to elastically couple the wheel suspension of a wheel on one side of the motor vehicle with the wheel suspension of the corresponding wheel of the same axle on the other side of the motor vehicle. The coupling is performed such that when one wheel is deflected inwardly, the spring action of the other wheel is also acted on in the direction of inward deflection. As a result, the lateral slope of the motor vehicle toward the outside of the curve is reduced during the travel of the motor vehicle in a curve, partly because the wheel suspension of the particular wheel that is the outer wheel in the curve is additionally supported by the spring system of the wheel suspension of the wheel that is the inner wheel in the curve and partly because the wheel suspension of the wheel that is the inner wheel in the curve is forced somewhat in the direction of inward deflection relative to the chassis of the vehicle.
By contrast, the roll bar shall not possibly affect the spring action characteristics of the vehicle during straight-line travel.
However, if the pavement is so uneven that one wheel on one side of the vehicle is forced in the inward deflection direction, while the corresponding wheel on the other side of the vehicle must be moved in the outward deflection direction to maintain the desired road contact, the driving smoothness is compromised by a roll bar, because the roll bar tends to counteract mutually opposite movements of the wheel suspensions coupled by the roll bar relative to the vehicle body. Thus, during straight-line travel, a roll bar may undesirably cause vibrations of one wheel to be transmitted to the opposite wheel of the same axle, which compromises the driving smoothness.
This contradiction between the safety and comfort requirements imposed on a roll bar can be eliminated if the roll bar is switched off during straight-line travel and is again switched on automatically during travel in a curve. Further improvement can be done if the roll bar can actively control the elastic coupling between the wheels.
The drawback of the prior-art roll bars with active torsion bar is that the switching on (coupling) of the roll bar during travel in a curve must take place, in general, automatically and very rapidly for safety reasons, because the vehicle could otherwise become uncontrollable in the curve. The high costs of the prior-art rollbars which are associated with these requirements on the actuator have caused that switch-on roll bars (also known under the name “active roll bars”) are not used in models manufactured in large series (i.e., in vehicles manufactured in large numbers).
Dynamic vibration control can also be beneficial with hand-held power tools with an impact drive in particular, such as rotary hammers, chisel hammers, and the like, where the hand-held power tool may be subjected to considerable vibrations. When these vibrations are transferred to a handle that is used to press the hand-held power tool against a work piece, the operator perceives the vibrations to be uncomfortable, and long-term exposure thereto may even result in injury. For this reason, double-shelled housings, with which the entire hammer is suspended in an outer shell such that it is resilient in its working direction, have usually been used to provide linear vibration damping of rotary hammers. This design is relatively expensive and do not reduce the vibration to a comfortable level.
Based thereon, one object of the present invention is to improve a hand-held power tool of the type described initially such that the amount of vibration on a handle that is decoupled from the tool is significantly reduced by the use of a MR fluid actuator unit.